1. Field of the Invention
The present invention relates generally to laser scanning apparatus and, more particularly, is directed to a laser scanning apparatus which displays a television picture or the like by using a laser beam.
2. Description of the Prior Art
A laser scanning apparatus is known, which displays, for example, a television picture by raster-scanning an intensity modulated laser beam in the horizontal and vertical directions in a two-dimensional manner. As a laser beam scanning system of this laser scanning apparatus, there is proposed a mechanical laser beam scanning system that employs a rotary polygon mirror for horizontal scanning. This mechanical system using the rotary polygon mirror, however, has the following unavoidable shortcomings:
(i) vibration occurs due to shaft whirl of the motor which rotates the rotary polygon mirror;
(ii) noise is produced due to sound emanated by the rotation of the rotary polygon mirror;
(iii) durability of the bearing of the rotary polygon mirror causes a problem;
(iv) since the rotation of the rotary drive motor is not started readily, the mechanical laser beam scanning system can not be started quickly; and
(v) since the rotation of the rotary drive motor is not so precisely stabilized, jitter occurs in the displayed image.
FIG. 1 is a schematic diagram showing an example of a raster-scanning type laser display apparatus which uses acousto-optic deflectors for both horizontal and vertical scannings.
Referring to FIG. 1, a laser beam emitted from a laser light source 1 is converged into a small spot size by a laser beam focusing lens 2 and is fed to a known type of acousto-optic light intensity modulator 3. This acousto-optic light intensity modulator 3 changes the intensity of a supersonic wave applied to an acousto-optic medium and thereby changes the efficiency with which a laser beam is deflected by the supersonic wave to thereby modulate the light intensity of the deflected laser beam.
A video signal, though not shown, is supplied to this acousto-optic light intensity modulator 3, whereby the laser beam is intensity-modulated in response to the video signal supplied thereto. The thus intensity-modulated laser beam emitted from the acousto-optic light intensity modulator 3 is supplied to a collimator lens 4, in which it is collimated to provide a laser beam of predetermined diameter. This laser beam is then fed to a horizontal scanning acousto-optic deflector 5.
The acousto-optic deflector 5 is comprised of an acousto-optic medium 5a and a supersonic wave generator 5b attached to one surface of the acousto-optic medium 5a. The supersonic wave generator 5b effectively utilizes the so-called piezoelectric effect. When a voltage is applied to the supersonic wave generator 5b from a high frequency generator 5c, a progressive wave of supersonic vibration is generated in the acousto-optic medium 5a. If the oscillation frequency of the high frequency oscillator 5c is low, then the acousto-optic medium 5a generates a progressive wave of long wavelength. Whereas, if the oscillation frequency of the high frequency oscillator 5c is high, the acousto-optic medium 5a generates a progressive wave of short wavelength.
The laser beam supplied to the acousto-optic deflector 5 is introduced into the acousto-optic medium 5a, wherein it encounters the progressive wave of supersonic vibration, is diffracted and then deflected by this progressive wave. At that time, the shorter the wavelength of the progressive wave of the supersonic vibration, the more the laser beam is deflected. Therefore, if the oscillation frequency of the high frequency oscillator 5c is repeatedly swept from a low frequency to a high frequency in a sawtooth wave fashion, then the laser beam emitted from the acousto-optic deflector 5 is deflected to provide a horizontal scanning laser beam which repeats the deflection scanning.
In order to project an image of high resolution on a screen, it is necessary to increase the diameter of the laser beam incident on the acousto-optic deflector 5. Nevertheless, although the diameter of the laser beam is large, the transmission speed of the progressive wave of supersonic vibration in the acousto-optic medium 5a is finite so that the frequency of the supersonic wave which encounters the laser beam is different in the incident laser beam at the portion near, as opposed to the portion distant, from the supersonic wave generator 5b. Thus, the deflection angle is changed with the position of the beam diameter. More specifically, in the portion near the supersonic wave generator 5b, the laser beam encounters a supersonic wave of high frequency and is considerably deflected, while in the portion distant from the supersonic wave generator 5b the laser beam encounters a supersonic wave of low frequency and is deflected by a small amount. Consequently, the laser beam is not collimated but converged and is then deflected as though it had traveled through a cylindrical lens. This is referred to as a so-called cylindrical lens effect.
The laser beam emitted from the acousto-optic deflector 5 and which becomes the horizontal scanning laser beam that repeats the deflection scanning is supplied to a correcting cylindrical lens 6. The correcting cylindrical lens 6 corrects the above-mentioned cylindrical lens effect and therefore the laser beam, which travels through the correcting cylindrical lens 6, becomes again a parallel laser beam which repeats the horizontal deflection scanning.
The horizontally deflected laser beam which has traveled through this correcting cylindrical lens 6 is supplied to a vertical scanning acousto-optic deflector 7. The acousto-optic deflector 7 has the same structure as the horizontal scanning acousto-optic deflector 5. To be more concrete, a supersonic wave generator 7b utilizing a piezoelectric effect is attached to one surface of an acousto-optic medium 7a and a voltage from a high frequency oscillator 7c is applied to the supersonic wave generator 7b, whereby a progressive wave of supersonic vibration is produced in the acousto-optic medium 7a. Then, the oscillation frequency of the high frequency oscillator 7c is repeatedly swept from a low frequency to a high frequency in a sawtooth wave fashion, thereby causing the incident laser beam to be repeatedly deflected in the vertical direction.
The number of times which the scanning laser beam is repeatedly deflected in the vertical direction is, for example, 60 per second which is small as compared with the above-mentioned horizontal deflection. Therefore, the cylindrical lens effect in the acousto-optic deflector 7 can be neglected.
Then, the laser beam emitted from the acousto-optic deflector 7, and which is repeatedly deflected in the horizontal and vertical directions in a two-dimensional fashion, is projected through magnifying projection lenses 8 and 9 to a screen 10. The magnifying projection lenses 8 and 9 are used in order to magnify the picture size to a practical screen size. The picture size on the screen 10 would otherwise be small due to the fact that the deflection angle of the deflector is as small as about 2 degrees.
As described above, the laser beam scans the screen 10 in a so-called raster scanning fashion and a video signal is supplied to the light intensity modulator 3 in synchronism with the horizontal and vertical deflections, thus making it possible to display a television picture.
As compared with a mechanical type laser display apparatus, the laser display apparatus utilizing the above-mentioned acousto-optic deflector is free from vibration and noise and is excellent in durability. Further, this type of laser display apparatus is quick to start and is free from time jitter components. Furthermore, this laser display apparatus utilizing the acousto-optic deflector is small in size and light in weight as compared with a mechanical type laser display apparatus.
In the acousto-optic deflector 5, the supersonic wave generator 5b is bonded to one surface of the acousto-optic medium 5a to produce a progressive, supersonic wave in the acousto-optic medium 5a, whereby the laser beam incident on the acousto-optic medium 5a is diffracted by the supersonic wave to change the direction in which the laser beam is emitted.
The higher the supersonic wave's frequency becomes, i.e. the shorter the supersonic wave's wavelength becomes, the larger the angle of the deflected light becomes. Accordingly, when the laser beam is scanned in a raster-scanning fashion by the acousto-optic deflector 5, the frequency of the supersonic wave must be swept from, for example, a high frequency to a low frequency and vice versa.
Although the sweep width .DELTA.f of the frequency of the supersonic wave must be increased in order to obtain a large deflection scanning angle, the attenuation of the supersonic wave becomes remarkable in the high frequency band. Therefore, this technique is not useful in actual practice and has a limit. Generally, the scanning angle is very small, e.g. about 2 degrees. Accordingly, when a picture is displayed by this type of laser display apparatus, this scanning angle must be magnified and projected by using the projection lenses 8 and 9. In that case, the spot size is also magnified in accordance with the magnifying ratio so that the resolution of the picture, i.e., the number N of resolvable beam spots per scanning line is not related to the magnifying ratio and is determined as: EQU N=.pi.D.DELTA.f/2 v cos .theta.
where D is the spot size of the laser beam, v is the sound velocity at which the supersonic wave proceeds in the acousto-optic medium 5a and .theta. is the angle formed by the scanning central angle and the incident laser beam.
Accordingly, in order to obtain a picture of high resolution, i.e. a high figure of N, the spot size of the laser beam incident on the acousto-optic medium 5a must be increased. There is then the substantial disadvantage that, if the spot size of the laser beam is increased, the scanning speed is decreased. This is because it takes a lot of time for the supersonic wave to propagate within the spot size. Assuming that the time necessary for switching the deflection direction is t, then the time t is expressed as: EQU t=.pi.D/2 v cos .theta.
As a result, the time t is increased in proportion to the spot size. That is, high speed scanning and a display of high resolution are contrary to each other.
Consequently, according to the laser display apparatus utilizing the prior-art acousto-optic deflector, there is the limit that only a picture of, for example, an NTSC system in which the horizontal resolution is 320 lines and the scanning lines 525/frame can be displayed. As a result, the laser display apparatus utilizing the acousto-optic deflector for horizontal scanning can not be applied to a high definition television receiver system having, for example, 1125 scanning lines.